Weathering is not essential as your tent will weather naturally with the first couple of uses/rain showers. If however you have the time and the inclination then it certainly wont do any harm.
What is weathering?
Brand new canvas tents may leak a tiny amount (half a cup in heavy rain, often not at all) This is perfectly normal. Cotton tents need to go through a weathering process to become watertight, it is the same for all bell tents and all canvas tents.
This is because the hole the needle makes for stitching the seams will be larger than the thread. The tiny holes in the seams need to expand around the thread in order to seal the seams completely (the thread itself will also expand inside the hole) This will happen when the seams become wet and then dry.
Weathering your bell tent.
By far the best method for weathering is by hosing the bell tent down. This is the method favoured by nearly all sellers and manufactures as it means that the tent will not be under tension when set up so will allow for the weathering process to be most effective.
Do not use a power hose as this may damage the waterproofing
Choose a good run of hot settled weather to start the process. Partially set up the bell tent, peg the ground sheet, insert the main pole and A frame and loosely peg the guy lines, enough to ensure that there is no possibility of the tent falling over but not so much that there is any tension of the seams.
Go round the bell tent hosing it down with a light sprinkler hose, it is important to focus on the door area and seams.
Allow the tent to dry fully after soaking to allow the threads in the seams to expand and to ensure the tent is not packed away wet. Done!